I have just fitted a new bath and plan to tile around it using a sealkit that fits under the tiles and covers the rim of the bath (a kind of "L" shape, if you understand what i mean), the problem is that at one end of the bath there is a gap of about 40mm between the wall and the bath, the original bath was slightly shorter and the gap was filled with a piece of painted fibre board and sealed with silicone (which looked really ugly). Is there any type of off the shelf sealing solution for this without having to use wood. Any help on this would be appreciated.

There is no off the shelf solution. The obvious solution is a piece of 40mm batten fixed to the wall to fill the gap.. Keep it about 5mm low so that you can fix a 40mm strip of tile to finish flush with the edge of the bath. Then carry on with your sealing strip. Hope this helps. The batten will need to be sealed with two coats of undiluted PVA See our project on PVA to allow the tiles to stick properly to the timber. Also see our project on Leaking shower or bath.